This invention relates generally to RF amplifiers operable at full power and at a lower back-off power, and more particularly the invention relates to improving the amplifier efficiency at a back-off power level.
FIG. 1 is a schematic of an RF/microwave amplifier including a bipolar transistor 2 having a grounded emitter, a DC biased RF input connected to the base, a power terminal (+V) connected through inductor 4 to the collector, and an output coupled to the collector through capacitor 6.
An RF and microwave amplifier can achieve high efficiency at full output power. However, in many wireless communication applications such as CDMA, the amplifier is operated at a back-off power level much more frequently than at full power level. Unfortunately, at back-off power level the efficiency of an amplifier usually suffers. The efficiency degrades as a result of not fully using the available voltage and current, as illustrated in the current versus voltage plot of FIG. 2 for a RF power amplifier operating at various input levels. Line 10 (solid) is the load line for the class A RF amplifier operating at full voltage, while line 12 (wavy) is the load line for the amplifier operating at a back-off power level. Changing from class A bias to class B bias helps the efficiency as shown by load line 14 (wavy), but efficiency at the back-off power level is still poor.
One method for achieving a higher efficiency at a back-off power level is to change the bias voltage with power level as shown in the plot of FIG. 3. At the back-off voltage as well as at full power, neither voltage nor current utilization is wasted. However, at the back-off voltage, the knee of the characteristic current/voltage curves degrades the amplifier efficiency.
The present invention is directed to improving the operating efficiency of an RF/microwave amplifier when operating at a back-off power level.
In accordance with the invention, efficiency of a power amplifier is increased at a back-off power level by increasing the load impedance of the amplifier at the reduced output power level as compared to load impedance at a higher power level including full output power.
In a preferred embodiment, an RF/microwave amplifier which is operable at two or more output power levels comprises a first signal amplification unit having an input terminal for receiving an input signal, an output terminal for producing an amplified output signal, and a power terminal for receiving electrical power. A load is coupled to the output terminal and has a first impedance value for a full power output and a second impedance value for a back-off power level, the second value being greater than the first value. The signal amplification unit may comprise at least one transistor such as a bipolar transistor or a field effect transistor.
In one embodiment of the invention the RF amplifier includes a second signal amplification unit in parallel with the first signal amplification unit with a second load coupling the output terminal of the second signal amplification unit to the RF output, the second load having a higher impedance than the first load, and a switch means for alternately connecting the first signal amplification unit and the second signal amplification unit to the RF output.